Large costume accessories can often be expensive to purchase custom-made, and their mass-produced counterparts are often no more professional looking than what you can craft much more cheaply at home. For a fish costume, you will need a large fin that attaches to the back of the body by a shoulder harness, which can be made by applying the same technique often used to make wings. To make your own costume fish fin for Halloween, a play or a costume party, follow these instructions.
Determine what style of fish fin you would like to make for your fish costume. Look at different types of fins to use as a reference (see Resources). On a large sheet of paper, draw out the shape of your fin in the size that you would like it to be. Make sure to take into account the size of the person wearing the costume, as a too-large fin may be difficult for him to wear. Add 2 extra inches to the bottom length. Cut the shape from the paper.
Lay the fin pattern over a sheet of transparent vinyl fabric and cut the vinyl around the edges of the paper. Cut a second, identical piece from the vinyl. Also lay the fin pattern over a length of sheer gauzy fabric, such as a shimmery organza or any other costume fabric that complements the type of fish your costume is emulating. Cut a copy of the pattern from the sheer fabric. Make a sandwich with the gauzy fabric between the two pieces of vinyl and pin the edges together.
Cut a length of the 16-gauge galvanised anchor wire that fits the bottom length of the pattern, subtracting 1 inch. Cut at least five more lengths of wire that fit the pattern along the width like spines, subtracting 1 1/2 inches from the measurement. These pieces will provide structure for your fin.
Take the galvanised wire pieces outside and spray paint them a similar colour to your gauzy inner fabric. Let the spray paint dry completely. On the fabric, create channels for the galvanised wire. Stitch one channel along the bottom, 2 inches above the bottom edge, then a vertical channel above that for each piece of wire that you cut. Insert the wire pieces into the channels and stitch along the open edge of the whole fin, closing the channels.
Create a harness for your fin out of the vinyl. Cut two squares of vinyl at least 5 inches by 5 inches and stitch them together. Open up the 2 extra inches on the bottom edge of the fin piece and stitch each edge to the centre of the vinyl square, anchoring it. Cut four rectangles of vinyl about the length of a backpack strap and stitch them into two rectangles. Stitch the short end of one rectangle to the top and bottom of one side of the anchor piece, then the other rectangle to the other side, creating a harness that you can slip your arms through. If necessary, add a horizontal strip of vinyl across the chest for extra stability.